The present invention relates to a device and a method for substantially enhancing the cooling of electric machines, e.g., motors and generators, to provide greater power density and greater efficiency and, more particularly, to a device and a method for passively cooling the stator winding end-turns of an electric machine by ebullition, which is to say, by the cyclical process of evaporization and condensation of a coolant.
Conventional cooling of electric machines, e.g., motors and generators, especially induction-type machines, typically involves a coolant-filled, highly conductive, outer cooling jacket, which encases the stator backiron element. In such systems, heat passes through the stator backiron in a radial path to the cooling jacket, which contains a fluid passage, e.g., a plurality of channels or tubes, through which cooling liquid flows. The cooling jacket, typically, is fabricated out of highly conductive material, e.g., aluminum. As additional measures, axial fans and oil fed through the rotor element may also be used to cool the machine. Such a system, however, overlooks the winding end-turns, where temperatures sometime are the highestxe2x80x94depending on the machine topology and application. This is particularly true for stator winding end-turns. Consequently, heat from the stator winding end-turns travels first to the stator backiron and then to the cooling jacket. Such a scheme is not an efficient means of transferring heat from the highest heat-producing source in the machine.
It is well known to those skilled in the art that conventional electric machines and other electrical devices can be cooled by ebullition. Indeed, for a conventional induction machine, heat produced in the stator core routinely is transmitted to a heat sink or cooling jacket by phase change cooling. See, for example, U.S. Pat. No. 3,710,156. Phase change cooling systems typically are passive; no additional energy input is required for the cooling system to function. Such systems conceptually are simple. Conventional phase-change cooling systems, however, also overlook providing a direct thermal link between the high temperatures generated at the stator winding end-turns and the cooling jacket.
Currently, there exists a need for an economical electrical machine that provides greater power density (measured in terms of kilowatts per kilogram (kW/kg)) at a lower cost (measured in terms of dollars per kilowatt ($/kW)). State-of-the-art high-performance induction machines have pushed power density to approximately 1.0 kW/kg and a cost per unit power to at least $8/kW.
One method for obtaining these objects of greater power density and lower cost is by lowering the overall thermal resistance of the electric machine. By doing this, either the density of the electrical current in the wires can be increased for a given winding temperature, or the winding temperature can be decreased for a given electrical current in the wires. The former leads to smaller motor size and weight for a given motor output, which translates into greater power density and lower cost. The latter reduces input power for a given output power, which leads to lower cost.
Thus, a solution to the shortcomings of the prior art is reducing the weight of conventional machines by providing a more efficient means of cooling the machine where the temperature gradients typically are the highest, i.e., in the stator winding, particularly in the end-turns.
In this setting, it would be desirable to provide a method of and a device for if cooling an electric machine that provides greater power density at a lower per unit power cost. Furthermore, it would be particularly desirable to provide such a method and device that reduce the size and weight of the machine in comparison to prior art devices.
The present invention features a method of and a device for substantially enhancing the cooling of a machine that is made more efficient by hermetically enclosing the stator, including the winding end-turns, and partially wetting them in a liquid, phase-change coolant or mixture of phase-change coolants, which flows in a self-sufficient thermodynamic cycle between the winding end-turns and a conventional cooling jackets. The liquid phase-change coolant boils and evaporates as the coolant absorbs heat generated by the inefficiencies of the machine stator end-turns. The hot, vaporized coolant collects at a cooling jacket where it cools and condenses. As the vaporized coolant condenses, heat is transferred, or rejected, to the cooling jacket, which contains water, refrigerant or some other heat-transferring coolant. The liquid condensate precipitates into a wicking matrix, which provides a liquid flow path for delivering the condensed liquid back to the winding end-turns. Capillary pressure head, or xe2x80x9ccapillarityxe2x80x9d, created by surface tension at the liquid-vapor interface in the winding end-turns provides the pressure head to pump the flow. The phase change cycle then repeats itself.
In comparison to typical prior art electric machines, by providing phase-change cooling at the winding end-turns in accord with preferred embodiments of the present invention, the machine mass can be reduced preferably by at least about 33 percent, thereby preferably providing at least about a 50 percent increase in power density. Moreover, the cost per unit power preferably can be reduced by at least about 20 percent and most likely more. Hence, electric machines can be provided with greater power density and greater efficiency more economically.
Preferred embodiments of the present invention provide a smaller, lighter, and more economical electric machine.
Certain preferred embodiments of the present invention can provide increased electrical current, ergo power density in an electric machine.
Additionally, preferred embodiments of the present invention can provide improved operating efficiency of an electric machine.
Thus, the present invention provides an electric machine comprising winding end-turns and having at least one hermetically sealed chamber surrounding the winding end-turns. The winding end-turns, which are bundles of wires that are wound into and out of slots in the stator armature and that extend axially from the armature, are partially wetted in a phase-change coolant. The liquid, phase-change coolant boils and vaporizes at or near the desired operating temperature of the winding end-turns. The vaporized coolant exits the end-turn windings and travels to a conventional, highly-conductive cooling jacket, which preferably also serves as a heat sink for the stator backiron. At the cooling jacket, the coolant vapor cools, thereby rejecting heat to the cooling jacket, and condenses on the inside of the cooling jacket. The liquid condensate coalesces into a wicking matrix, which is located around the winding end-turns and between the winding end-turns and the cooling jacket. The interaction between the solid winding end-turns, the coolant by liquid, the coolant vapor and the interface between the coolant liquid and the coolant vapor results in a pressure head by capillarity that provides liquid coolant to the winding end-turns when and as needed.
Other aspects and embodiments of the invention are discussed below. Moreover, additional advantages of the present invention are apparent from the drawings and specifications that follow.